In the latest twist of a long-running legal battle, the Economic Community of West African States court in Nigeria's capital Abuja said the "state of Senegal did not violate any of (Sonko's) rights."
It comes as the Supreme Court in the Senegalese capital, Dakar, hears another case relating to Sonko's hopes of running for president.
Sonko has been locked in a fierce tussle with the state for more than two years and says authorities are trying to scupper his bid to contest Senegal's February 2024 presidential election.
Last month, a provincial judge ordered Sonko's reinstatement on the electoral lists after authorities removed him.
The Supreme Court will hear an appeal from the state challenging that ruling.
Sonko, who came third in the 2019 presidential election, was in June found guilty of morally corrupting a young person and sentenced to two years in prison. He was not present at the trial -- which he denounced as a plot to torpedo his political career -- and was convicted in absentia.
In late July, he was imprisoned on a fresh string of charges, including fomenting insurrection, criminal association in connection with a terrorist enterprise and undermining state security over incidents dating back to 2021.
His conviction in June sparked clashes that left 16 dead, according to the government, or as many as 30, according to his party.